Electronic annunciator circuit



United States Patent 3,525,988 ELECTRONIC ANNUNCIATOR CIRCUIT Kenneth C. Linder, Arlington Heights, Ill., assignor to The Scam Instrument Corporation, Skokie, Ill., a corporation of Illinois Filed Sept. 5, 1967, Ser. No. 665,433 Int. Cl. G08b 23/00 US. Cl. 340213.1 12 Claims ABSTRACT OF THE DISCLOSURE A compact, self-contained, low power annunciator circuit operable directly on 120 volt line current and capable of energizing an electroluminescent annunciator panel upon receipt of an alarm signal. This circuit responds to an alarm with a flashing visual signal and to alarm acknowledgement with a steady visual signal. The simple circuit comprises an electroluminescent annunciator panel, a ringing coil and switching transistor for energizing the electroluminescent panel, a silicon controlled rectifier logic-rnemory circuit, a flash frequency multivibrator, an electroluminescent panel driving multivibrator, and an AC. to DC. power converter.

This invention relates to an improved annunciator system and, more particularly, to such a system made up of self-suflicient annunciator modules, each of which is capable of exciting its own electroluminescent annunciator panel in response to an alarm signal.

Alarm or annunciator systems are extensively used in a variety of industrial applications to monitor process con ditions, the operability of drive and control components, and numerous other variable factors. In general, these systems include annunciator or alarm means controlled by field contacts that respond to the condition to be monitored to provide audible and visible indications, usually of abnormal conditions. The alarm or annunciator and the controls therefor are frequently manufactured in modular form, having some form of indicator light associated with each module and having a central horn associated with all of the modules. Frequently, these systems are designed so that an alarm condition will cause an indicator light to flash and will also energize the central horn. A push button is usually provided with each module so that a supervisor can acknowledge his having noted an alarm. Depression of this acknowledgement push button usually stops the horn and also causes the indicator light to discontinue flashing. The indicator light will usually then remain in an illuminated state until the alarm condition is terminated.

Since failure of an annunciator can easily result in loss of lives and destruction of valuable property, it is important that these systems be extremely reliable. Because of this, transistors have come to replace the relays and magnetic amplifiers previously used in annunciator circuits, and electroluminescent annunciator panels have come to replace incandescent lamps as alarm indicators. Unfortunately, transistors require direct current for their operation, and electroluminescent panels require a source of high voltage, high frequency alternating current. Therefore, present day annunciators using transistors and electroluminescent annunciator panels generally require two external power supplies for operation. Failure of either power supply can easily disable an entire alarm system.

Accordingly, one object of this invention is to provide a new and improved alarm or annunciator circuit.

Another object is to provide an alarm or annunciator circuit including an electroluminescent indicating means which requires no external source of high frequency alternating current for its operation.

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Another object is to provide an electroluminescent annunciator circuit in which the switch controlling the operation of an electroluminescent panel is never subjected to a reverse bias, so that a transistor or other similar nonsymrnetrical switching means can be used for switching.

A further object is to provide an annunciator circuit whose operation is independent of any external device with the exception of the 60 cycle line, so that failure of any one unit in a system will not affect the operation of the remaining units.

Another object is to incorporate silicon controlled rectifier logic in an annunciator system, thereby decreasing the number of parts, simplifying the design, and improving reliability.

A further object of this invention is to provide means whereby the first circuit in a given set of circuits to be activated by an alarm can force all circuits responding at a later time to respond in a different manner, thus making it easy to determine which annunciator module was activated first.

In accordance with these and many other objects, an embodiment of the invention comprises a self-contained electroluminescent panel annunciator circuit operable on 120 volt alternating current and entirely self-sufficient in all other respects. The circuit contains two signal sourcesa high frequency source for electroluminescent panel excitation, and a low frequency source for flasher circuit eXcitation-so that no external signals are required for its proper operation. Two silicon controlled rectifiers, one responding to the commencement of an alarm, and the other responding to acknowledgement of an existing alarm, are used as logic elements controlling the interaction of the above-mentioned two signal sources with an electroluminescent panel indicating means. The electroluminescent panel is connected in parallel with a ringing coil and is driven by a simple electronic switch. This arrangement allows the electroluminescent panel to be eX- cited with a constant voltage power source.

The invention, together with other objects and advantages, will best be understood from considering the following detailed description in conjunction with the drawings in which:

FIG. 1 is a schematic circuit diagram of an annunciator or alarm system embodying the present invention;

FIG. 2 is a simplified diagram illustrating the circuit used to excite an electroluminescent panel indicator in the system shown in FIG. 1;

FIG. 3 is a simplified diagram illustrating the control circuits indicated in the system of FIG. 1; and

FIG. 4 is a schematic circuit diagram illustrating the manner in which three or more of the circuits shown in FIG. 3 can be interconnected.

Referring now to FIG. 1 of the drawings, an annunciator circuit characterized by the features of the invention is designated generally as 20. The circuit is powered by a volt alternating current power source 22. A diode 24 rectifies the incoming alternating current, and a capacitor 26 smooths the resulting direct current. The diode 24 is oriented so as to product a positive voltage on a line 28 and a negative voltage on a line 30. Clearly, there is nothing unique about this power supply arrangement, and other equivalent means could be used here.

The input to the annunciator circuit 20' consists of a set of alarm or field contacts 46 which either opens or closes in response to an alarm condition. When the alarm contacts are closed, the electrical node 47 is connected directly to the positive line 28 by the alarm contacts. When the alarm contacts are open, the same node 47 is connected to the negative voltage point 30 by a resistor 39. This node 47 is connected to a second node 49 by a simple NOT circuit 45, which might be a single transistor inverting amplifier. The output of this NOT circuit, and therefore the potential of node 49, is always the inverse of its input, which is the voltage appearing at node 47; if the voltage at node 47 is negative, the output at node 49 will be positive, and vice versa.

A switch 50 and a resistor 55 are used to couple the control electrode of a first silicon controlled rectifier 42 to one or the other of the two nodes 47 and 49. If the alarm or field contacts 46 are normally open, closing only in response to an alarm condition, then connection is made to node 47, and the NOT circuit 45 is bypassed. If the alarm or field contacts 46 are normally closed, then connection is made to node 49. In either case, an alarm condition Will result in a positive voltage being applied to the control electrode of the first silicon controlled rectifier 42. This tfirst silicon controlled rectifier 42 is normally in a nonconducting state. Therefore, the occurrence of an alarm will cause this first silicon controlled recticfier 42 to conduct, and its anode potential (a node 68) will drop to a level roughly equivalent to the potential of the negative line 30.

The annunciator circuit 20 contains a second silicon controlled rectifier 48 which is normally maintained in a conductive state by current flowing to its control element or gate from the normally positive node 68 through a resistor 51. When the potential at node 68 drops in response to an alarm condition, the sustaining current flowing through the resistor 51 is cut off. Simultaneously the second controlled rectifier 48 is rendered nonconductive by a negative pulse which is transmitted from the node 68 to the anode of this second controlled rectifier 48 by a capacitor 52. Thus an alarm condition causes the first silicon controlled rectifier 42 to conduct, and simultaneously renders the second silicon controlled rectifier 48 nonconductive.

A resistor 40, which couples the anode of the first silicon controlled rectifier 42 to the positive line 28, is of a very large ohmic value and is incapable of supplying sufiicient anode current to maintain the first controlled rectifier 42 in a conducting state in the absence of gate current. Gate current is supplied to the first controlled rectifier 42 only when an alarm condition is actually present; therefore, if no other source of anode current is supplied, the first controlled rectifier 42 will turn off immediately upon the termination of an alarm condition. To prevent this from happening, the first silicon controlled rectifier 42 is supplied with a second source of anode current which flows through two resistors 54 and 56, and through a diode 58. This second current path supplies enough current to sustain the first silicon controlled rectifier 42 in a conducting state whether any gate current is present or not. This second current path is interrupted, however, when the second silicon controlled rectifier 48 is in a conducting state. When the second silicon controlled rectifier 48 conducts, it connects anode 60 to the negative line 30, and this in turn back-biases the diode 58 and breaks the second current path. When the first silicon controlled rectifier 42 goes into conduction in response to an alarm condition, it continues to conduct at least until the second silicon controlled rectifier 48 is returned to conduction; and if the alarm condition still persists past that time, it continues to conduct until the alarm condition is terminated.

The control element or gate electrode of the second silicon controlled rectifier 48 can be connected to the positive line 28 by a series circuit including a resistor 66 and a manual switch 64. The purpose of this circuit is to provide means whereby an alarm signal can be acknowledged by a supervisory employee. Operation of the switch 64 causes current to flow in the control electrode of the rectifier 48 and places it in a conductive state. The resistor 54 coupling the anode of the rectifier 48 to the positive line 28 is of a small enough ohmic value to sustain conduction of silicon controlled rectifier 48 even after the acknowledgment switch 6-4 is opened.

In summary, then, the voltage at node rises in re sponse to an alarm condition, and falls in response to acknowledgement of the larm condition; the voltage at node 68 falls in response to an alarm condition, and rises either in response to acknowledgment of the alarm condition, or, if the alarm persists after acknowledgment, upon termination of the alarm condition.

A switch 62 is supplied in the circuit 20 which, when closed, permanently deprives the first silicon controlled rectifier 42 of conduction sustaining current. When this switch 62 is closed, diode 58 is back-biased, and the second current path mentioned above is permanently broken. This means that the first silicon controlled rectifier 42 will turn off whenever an alarm condition terminates, with no need for acknowledgment. When this switch 62 is closed, both silicon controlled rectifiers will return to their original states immediately upon termination of an alarm condition. No acknowledgment is required. This mode of operation is often used during warm-up or maintenance periods when many nuisance alarms are expected and when acknowledgment of each alarm could serve no useful purpose.

In accordance with the present invention, the annunciator circuit 20 includes an electroluminescent panel 32 which is connected in parallel with an inductor 34. Referring now to FIG. 2, this parallel combination of the panel 32 and the inductor 34 is connected in series with a transistor switch 36, and the resulting series circuit is connected between the positive line 28 and the negative line 30. When the panel is to be illuminated, a high frequency fluctuating signal is supplied to the base of the switching transistor 36. The frequency of this fluctuating signal has been previously carefully chosen to correspond to that frequency needed for proper excitation of the electroluminescent panel. It will be found that this simple circuit is capable of generating a very large amplitude alternating current waveform across the electroluminescent panel 32.

The base of the transistor 36 is not directly connected to a source of high frequency signal, but is connected to the output of a negative AND or NAND gate 38. This particular NAND gate gives a more positive output if and only if all of its input terminals are negative. It can consist of a simple diode AND gate followed by a one-transistor inverting amplifier, for example. The NAND gate 38 shown has three separate inputs, one of which 44 is coupled to a 400 cycle source of panel excitation voltage. In order for the 400 cycle signal 44 to pass through the NAND gate 38, the remaining two inputs of the NAND gate 41 and 43 must both be negative.

The first of these two remaining NAND gate inputs 43 is labeled control input. This input, which is connected to the node 68, is used as an ultimate control over annunciator panel illumination. When this input is positive, the fluctuating signal applied to the input 44 cannot reach the electroluminescent panel 32, and the panel remains dark. The remaining input 41 is connected to a controllable source of extremely low frequency fluctuating signal. Since the panel 32 is dark whenever the input 41 is positive, the presence of a low frequency signal at this input will cause the panel to flash on and off at the low frequency rate, if the control input 43 is negative.

Referring now to FIG. I, it can be seen that the annunciator circuit 20 contains two multivibrator signal sources 70 and 72. Each of these multivibrator circuits, which circuits are of conventional design, requires a source of direct current for its operation, and each generates a square waveform. Both multivibrators are connected to the negative line 30; these connections were omitted to,

simplify the drawing. The high frequency multivibrator 72 draws its operating voltage from the node 74, the potential of which is determined by the value of two resistors 76 and 77, directly connected across the lines 28 and 30. The high frequency multivibrator 72 operates continuously. The low frequency multivibrator 70 draws its operating voltage from the node 60. As set forth above, a positive voltage appears at node 60 immediately upon the commencement of an alarm condition, and that positive voltage remains until the alarm condition is acknowledged by the depression of the push button 64. Thus, the low frequency multivibrator operates only during those periods when the node 60 is positive. At all other times the low frequency multivibrator output will be negative, since it will have no connection to any positive source of potential.

The control input to the NAND gate 43 is connected to the node 68. The second and third inputs to the NAND gate 41 and 44 are connected respectively to the two multivibrators 70 and 72.

In the absence of any alarm signal a positive voltage exists at node 68. This voltage blocks any other signals from flowing through the NAND gate and effectively prevents the electroluminescent annunciator panel 32 from being illuminated. When an alarm condition arises, the voltage at the node 68 immediately falls and a negative voltage is applied to NAND gate input 43. Simultaneously the voltage at node 60 rises, and the low frequency multivibrator begins to function, its output being fed to NAND gate input 41. The high frequency multivibrator continues to function, its output being fed to NAND gate input 44. As shown above, this combination of input signals will cause the electroluminescent annunciator panel 32 to flash on and off at the low frequency multivibrator rate, indicating the presence of an unacknowledged alarm condition. When the alarm condition is acknowledged, the voltage at node 60 falls, disabling the low frequency multivibrator. As shown above, the output voltage of the low frequency multivibrator will now be negative. Assuming that the alarm condition persists, node 68 will remain negative. Now the NAND gate inputs 41 and 43 are both negative, and input 44 still receives the high frequency signal. The electroluminescent annunciator panel 32 will now remain steadily illuminated, indicating that an acknowledged alarm condition still persists. When an acknowledged alarm condition finally terminates, node 68 becomes positive once again, and the resulting positive voltage applied to the NAND gate control input 43 once more prevents the electroluminescent annunciator panel 32 from being illuminated. The circuit is now ready for the next alarm.

Occasionally, when annunciator circuits of the type described above are used in large numbers to monitor many different alarm variables, it is desirable to have some way of indicating which of a series of alarms was the first to arise. An annunciator system capable of performing this task is said to be capable of giving a first out indication. In an annunciator system utilizing circuits of the type shown in FIG. 1, first out indications can be had by providing a way in which an alarm indication from the first annunciator to sense an alarm condition can be fed to all other annunciators within the same system and utilized to prevent annunciators responding to alarms arising at later times from giving out a flashing indication. The annunciator panel corresponding to the alarm that was first in time will be easily identifiable, since it will be the only panel that will flash on and off.

FIG. 3 is a fragmentary circuit diagram that illustrates a way in which the annunciator circuit of FIG. 1 can be adapted to first out indication service. The capacitor 52, which in FIG. 1 connected node 68 to node 60, does not connect to node 60 in FIG. 3. Capacitor 52 now connects instead to a new node 92. Node 92 is connected to the negative line by a resistor 80, to another new node 91 by a resistor 78, and to the node 60 by a series circuit composed of two high gain inverting amplifiers 84 and 86, and a second capacitor 88. The first inverting amplifier 84 gives a positive output at a node 95 when its input potential at the node 92 is more negative than the potential of the negative line 30; it gives a negative output at the node 95 when its input potential at the node 92 is equal to or more positive than the potential of the negative line 30. The second inverting amplifier 86 inverts the output of the first. Hence the second inverting amplifier gives a negative output at a node 96 when the potential of the node 92 is more negative than the potential of the negative line 30; it gives a positive output when the potential of the node 92 is equal to or more positive than the potential of the negative line 30. The amplifiers 84 and 86 could both be of the one-transistor grounded emitter type; alternatively, a single non-inverting amplifier could be used in place of the two inverting amplifiers shown.

In FIG. 3, the new node 91 is directly connected to an inhibit terminal 93. This same node 91 is also connected to the node 60 by a series circuit composed of a diode 90 and a switch 97. Assuming the switch 97 to be closed, it can be seen that a positive potential arising at node 60 will be transmitted to the node 91 and to the inhibit terminal 93 by the diode 90.

An annunciator system utilizing modules of the type shown in FIG. 3 is illustrated in FIG. 4. Three annunciator circuit modules are shown and labeled respectively module A, module B, and module C. Of course, the system could contain many more than three modules if desired. The parts in each module shown in FIG. 4 are numbered to correspond with the numbering in FIGS. 1 and 3.

FIG. 4 shows an inhibit line 99 which interconnects the inhibit terminals 93 of every module A, B, C within the system. Similarly, the negative lines 30 of every module within the system are interconnected; the negative line interconnection, however, was omitted from FIG. 4 to simplify the drawing. It is assumed, for the present, that all three of the switches 97 are closed.

Assume that the annunciator system is operating and that no alarms have been received. The silicon controlled rectifier 42 in each of the modules is non-conducting and the nodes labeled 68 are positive in potential with respect to the negative lines 30. The silicon controlled rectifier 48 in each circuit is conducting, and the nodes labeled 60 are approximately at the same potential as the negative voltage lines 30. The nodes labeled 92, all being connected to the negative lines 30 by the resistors 80, are also at the potential of the negative lines 30. The nodes labeled 91, and therefore the inhibit line 99, are at the potential of the negative lines 30 because of their connections to the nodes 92 my the resistors 78. A positive potential is present at node 96, as explained above. All the nodes numbered 68 or 96 are now positive in potential, while all nodes numbered 60 or 92 are negative in potential. Since a capacitor connects each node within the positive group of nodes to a node within the negative group of nodes, all of the capacitors shown in FIG. 4 will be appropriately charged.

Assume that an alarm condition is sensed by module A. The silicon controlled rectifier 42 in module A conducts to connect node 68 in module A to the negative line 30. The capacitor 52 in module A, being charged, forces the node 92 in module A to a potential more negative than that of the negative line 30. This, in turn, causes the potential of node 96 in module A to fall negative. The capacitor 88 in module A, also being charged, applies a negative pulse to node 60 in module A in exactly the same manner as the capacitor 52 in FIG. 1 applied a negative pulse to the FIG. 1 node 60 in response to an alarm condition. The silicon controlled rectifier 48 in module A ceases to conduct, and the potential of node 60 in module A now rises to a positive value, just as did the potential of node 60 in FIG. 1. The electroluminescent annunciator panel (not shown) associated with module A in FIG. 4 now responds in the same manner as did the panel 32 in the circuit shown in FIG. 1, i.e., it begins to flash.

The positive voltage at the node 60 in module A is transferred by the diode 90 and the closed switch 97 to the node 91 in module A and to the inhibit line 99. This positive voltage now appears at the node 91 in each of the three modules A, B, and C. The nodes labeled 92 in each module, being coupled both to the negative lines and to the positively biased nodes 91, now rise in potential to an intermediate voltage level. It is assumed that this intermediate voltage level of the nodes 92 has been chosen to be somewhat positive with respect to the potential of the nodes 68 when the silicon Controlled rectifiers 42 are turned off. All of the capacitors labeled 52 now acquire a reverse charge corresponding to the new potentials of the nodes labeled 92.

Assume that a second alarm condition is now sensed by module B. As in the case of module A, the silicon controlled rectifier 42 in module B now conducts and connects the node 68 in module B to the negative line 30. However, the charge on capacitor 52 in module B prevents the potential of node 92 in module B from falling below that of the negative line 30. Because of this, the potential of node 96in module B never goes negative, and the silicon controlled rectifier 48 in module B continues to conduct. Both node 60 and node 92 in module B are now negative. This corresponds to the state of the circuit shown in FIG. 1 when an alarm condition persists following acknowledgment. The electroluminescent annunciator panel (not shown) associated with the module B in FIG. 4 is therefore illuminated, but it does not flash. Similarly, if a third alarm condition is now sensed by module C, the panel associated with that module is illuminated, but it does not fiash.

Conditions are often encountered in the field that make it desirable to have some way of temporarily disconnecting a single annunciator from its usual first out group, and also of temporarily arranging for that same annunciator to reset itself, without acknowledgement, whenever a corresponding alarm condition is remedied. For example, it may not be desirable to have to acknowledge the frequent nuisance alarms which may occur when a device is first turned on, or when a device is operated in some other than its usual mode of operation for a short time. For this reason, each annunciator circuit of the type shown in FIG. 3 can be equipped with a ganged switch 97-62. When thrown, this switch disconnects the dioded 90 from the node 91 thereby removing the circuit from its usual first out group, and simultaneously closes the quick reset switch 62, whose function is explained above.

It is also desirable to have a horn respond to the existence of an alarm with an audible signal and to have this horn signal suppressed when the alarm is acknowledged. A horn control voltage can be conveniently obtained from the node 60. Referring to FIG. 1 and to FIG. 4, it can be seen that a diode 98 can be used to connect the node 60 in each circuit to a common horn line 101 which runs to a central horn 100. Whenever any one of the circuits responds to an alarm, it will apply a negative potential to the horn line 101, and this in turn will initiate the sounding of the central horn 100.

One final part of the annunciator circuit shown in FIG. 1 remains to be explained. A series circuit composed of a resistor 25 and a manually operable TEST switch. 27 connects the positive line 28 to the gate of the first silicon controlled rectifier 42. When the switch 27 is closed, current is supplied to the gate of the first silicon controlled rectifier, and the annunciator responds in the same way as it responds to an alarm condition. This series circuit provides a convenient way in which the annunciator circu-it can be tested for proper operation.

Although the present invention has been described with reference to an illustrative embodiment thereof, it should be understood that numerous other modifications and changes will readily occur to those skilled in the art, and it is therefore intended by the appended claims to cover all such modifications and changes that will fall within the true spirit and scope of the invention.

What is claimed as. new and desired to be secured by Letters Patent of the United States is:

1. An annunciator circuit comprising an electroluminescent annunciator panel connected in parallel with an inductor,

a direct current potential source for energizing the panel,

current switching means connecting the source to the parallel combination of the panel and the inductor,

a logic circuit having a plurality of inputs and an output which is connected to said current switching means for controlling the conduction of the switching means,

a high frequency source for generating a waveform of the proper frequency for driving the electroluminescent panel, the high frequency source being connected to one input of said logic circuit, and

means responsive to an alarm condition and connected to another input of said logic circuit to enable the logic circuit when an alarm condition arises.

2. The annunciator circuit set forth in claim 1 in which the logic circuit includes an AND circuit.

3. The annunciator circuit set forth in claim 1 in which the logic circuit includes three inputs and which annunciator circuit is further provided with a low frequency source of fluctuating signals coupled to the third input of the logic circuit.

4. The annunciator circuit set forth in claim 3 includmeans connected to the low frequency source for placing the low frequency source in operation in response to an alarm condition, and

inhibiting means for rendering the low frequency source ineffective during the persistence of an alarm condition.

5. The annunciator circuit set forth in claim 4 in which the inhibiting means includes a manually operable switch.

6. An annunciator circuit comprising:

means for generating a control voltage in response to an alarm condition;

a voltage source;

low frequency generator means energized by the voltage source;

high frequency generator means;

a logic circuit having three inputs which are coupled respectively to the low frequency generator means, the high frequency generator means, and the means for generating a control voltage;

an electroluminescent annunciator panel;

means coupling the output of the logic circuit to the electroluminescent annunciator panel; and

acknowledgement means for rendering the voltage source ineffective to energize the low frequency generator means in response to the acknowledgement of an alarm.

7. The annunciator circuit set forth in claim 6 in which the low and high frequency generator means are multivibrator means.

8. An annunciator circuit comprising a potential source,

a first silicon controlled rectifier having a control electrode,

first circuit means including a first resistance means connecting the first rectifier across the potential source, the first rectier normally being in a nonconductive state and the ohmic value of the first resistance means being large enough to prevent sustained conduction through the first rectifier in the absence of an enabling signal on its control electrode,

alarm responsive means coupled to the control electrode of the first rectifier for applying an enabling signal to the control electrode to place the first rectifier in a conductive condition when an alarm condition arises,

indicating means coupled to and controled by the first rectifier and energized when the first rectifier is placed in a conductive condition,

a second silicon controlled rectifier having a control electrode, said second rectifier normally being, in a conductive state,

second circuit means coupling the second rectifier across the potential source, said second circuit means also being coupled to the first rectifier for energizing the first rectifier to maintain conduction through the first rectifier when the second rectifier is in a nonconductive state,

third circuit means coupling the first and second rectifiers to place the second rectifier in a nonconductive state in response to conduction through the first rectifier, thereby permitting the second circuit means to maintain conduction through the first rectifier,

and means coupled to the control electrode of the second rectifier and including first switch means for placing the second rectifier in a conductive state to acknowledge an alarm indication, thereby preventing the second circuit means from further maintaining conduction through the first rectifier, and thereby rendering continued conduction through the first rectifier dependent on the presence of an enabling sig nal on the control electrode of the first rectifier.

9. The annunciator circuit set forth in claim 8 including a fourth circuit means connected to the second circuit means and including second switch means for inhibiting energization of the first rectifier by the second circuit means.

10. The annunciator circuit set forth in claim 8 includa logic circuit coupled to the indicating means for controlling its operation, said logic circuit having at least two inputs,

means coupling one of the inputs to the first rectifier,

and

means controlled by the second rectifier and coupled to the other input.

11. An annunciator system comprising an alternating current source of power;

a plurality of monitoring means for monitoring alarm conditions; and

a plurality of annunciator modules, each of which is coupled to one of the monitoring means and each of which comprises an electroluminescent panel,

an inductor connected to said panel,

circuit means controlled by the monitoring means and connected to the electroluminescent panel for supplying high frequency alternating current to the panel in response to an occurrence of the alarm condition, said circuit means requiring no external sources of power or signal for operation other than a single source of direct current, i and an alternating-to-direct current power converter connected to the alternating current source of power and to the circuit means, and supplying direct current to the circuit means, each module being thereby rendered self-sufiicient and capable of functioning so long as the alternating current source of power is maintained irregardless of malfunctions within other parts of the system. 12. An annunciator system comprising: a plurality of annunciator modules each including an inhibit terminal; a single inhibit line interconnecting the inhibit terminals of the annunciator modules; first and second bistable devices within each module having inputs and outputs; indicator means associated with each module; circuit means within each module connecting the first and second bistable device outputs to the indicator means for operating the indicator means in ditferent states in accordance with the states of the first and second bistable devices; alarm responsive means within each module connecting to an input of the first bistable device for setting the first bistable device in response to an alarm condition; coupling means within each module connected to the inhibit terminal and coupling the alarm responsive means to an input of the second bistable device for setting the second bistable device in response to an alarm condition, said coupling means being prevented from setting the second bistable device when the inhibit terminal is at a first potential level; and a diode within each module connecting an output of the second bistable to the inhibit terminal, said' diode being oriented and said output being arranged to clamp the inhibit terminal at the first potential level when the second bistable is set.

References Cited UNITED STATES PATENTS 3,076,185 1/1963 Ida 340-2131 3,264,626 8/1966 Mounce 340213.l 3,381,286 4/1968 Walsh 340-213.1

THOMAS B. I-IABECKER, Primary Examiner C. MARMELSTEIN, Assistant Examiner US. Cl. X.R- 

